So my scenario is this: I am writing a stack (using azure native library) to deploy a microservice into an azure environment where a Front Door instance has been set up by another stack. I want to add a backend objects, health probe, load balancer and routing rules for the microservice. Can I do that with Network.FrontDoor? Previous attempt by another dev assumed this wasn't possible so he did it by adding an AzurePowershell step in the pipeline. Would be much nicer to be able to do this via Pulumi

The front door would have to output the needed information for the work you doing. Then do a stack reference to the output in your working stack. https://www.pulumi.com/learn/building-with-pulumi/stack-references/

I know about stack references that's fine. But for example I can;t find an API to just add a new backend object to an existing Front Door instance

Just looked at ARM, appears Microsoft didn't expose a separate API for backends. It all nested within Front Door. https://docs.microsoft.com/en-us/azure/templates/microsoft.network/frontdoors?tabs=bicep